Since the Multiple Input Multiple Output (MIMO) technologies are important to an improvement of a peak rate, and a utilization ratio of system spectrums, all the radio access technology standards of Long Term Evolution (LTE), LTE-Advanced (LTE-A), etc., are built based upon the MIMO plus Orthogonal Frequency Division Multiplexing (OFDM) technologies. A performance gain of the MIMO technologies arises from a spatial freedom available in a multi-antenna system and a larger amount of data are transmitted due to the spatial freedom, so one of the most important evolvements of the MIMO technologies being standardized is an extension of dimensions.
In the LTE Release 8 (Rel-8), MIMO transmission of at most four layers can be supported. In the LTE Rel-9, primarily the Multi-User MIMO (MU-MIMO) technologies are enhanced, and at most four downlink transmission data layers can be supported in MU-MIMO transmission of the Transmission Mode (TM)-8. In the LTE Rel-10, an 8-port Channel State Information-Reference Signal (CSI-RS), a User Equipment (UE)-specific Reference Signal (URS), and a multi-granularity codebook have been introduced to further improve a spatial resolution of channel state information, and to further extend a transmission capacity of Single-User MIMO (SU-MIMO) to at most 8 transmission data layers.
In a base station antenna system structured as a Passive Antenna System (PAS), a plurality of antenna ports are arranged horizontally, where each antenna port corresponds to a separate radio frequency-intermediate frequency-baseband channel, and a plurality of array elements in a vertical dimension corresponding to each antenna port are connected through a radio frequency cable. Thus with the existing MIMO technologies, only spatial characteristics of signals of respective UEs in a horizontal dimension can be optimized by adjusting relative amplitudes and/or phases between the different ports in the horizontal dimension, and only uniform sector-level beam-forming can be made in the vertical dimension. After the Active Antenna System (AAS) technologies have been introduced to a mobile communication system, the base station antenna system can be provided with a higher freedom in the vertical dimension, and can optimize a signal at a UE level in the three-dimension space.
The MIMO technologies are being further advanced in the industry toward becoming three-dimensional and large-scale. There is a research project ongoing in the 3rd Generation Partnership Project (3GPP) on Three-Dimension (3D) channel modeling, and there are also researches and standardization efforts to be conducted as expected on the technologies of Elevation Beam Forming (EBF) of eight or less antenna ports, and Full Dimension MIMO (FD-MIMO) of more than eight ports (e.g., 16, 32, or 64). Researches and test efforts have been conducted from the foresighted perspective in the academic community on the massive MIMO technologies based upon a larger scale array of antennas (including one or more hundreds and even more array elements). The academic researches, and the results of the preliminary channel in situ measurements have showed that the massive MIMO technologies can greatly improve the utilization ratio of system bands, and support a larger number of accessing UEs, so the massive MIMO technologies are expected by the major research organizations to be one of the most promising physical layer technologies in a next generation mobile communication system.
The massive MIMO technologies are required to be applied to a massive array of antennas. Although the spatial resolution can be maximized, and the MU-MIMO performance can be optimized, using a fully-digital array, this structure necessitates a large number of Analog-Digital (AD)/Digital-Analog (DA) conversion elements and radio frequency-baseband processing channels, thus resulting in a considerable burden on both a device cost, and the complexity of baseband processing. This becomes particularly prominent in a high frequency band and a large bandwidth. In order to lower the implementation cost and the device complexity of the massive MIMO technologies, the hybrid digital and analog beam-forming technology has been recently proposed.
As illustrated in FIG. 1, the hybrid digital and analog beam-forming refers to Digital Beam-Forming (DBF) plus an additional level of beam-forming, i.e., Analog Beam-Forming (ABF), on a radio frequency signal proximate a front end of an antenna system. With analog beam-forming, a signal to be transmitted can be roughly matched with a channel simply. The dimension of an equivalent digital channel resulting from the analog beam-forming is less than the real number of antennas, so that the numbers of AD/DA conversion elements and digital channels, and corresponding complexity of baseband processing can be greatly lowered. Interference of residual analog beam-forming can be handled in the digital domain to thereby guarantee the quality of MU-MIMO transmission.
As compared with fully-digital beam-forming, hybrid digital and analog beam-forming is a solution with a tradeoff between the performance and the complexity, and has a promising application prospect in a system operating in a high frequency band and in a high bandwidth, or with a large number of antennas.
In the MIMO technologies, and particularly the MU-MIMO technologies, the precision of pre-coding/beam-forming, and the efficiency of a scheduling algorithm depends directly upon, and thus the performance of the system as a whole is affected by, the precision of channel state information obtained at the network side, so how to obtain the channel state information is one of the core issues in the standardization of the MIMO technologies.
In the existing LTE signal structure, a channel can be estimated according to a reference signal in a baseband signal to obtain a channel state required for digital beam-forming. Since a plurality of analog channels in the part of analog beam-forming are equivalent to a digital channel, the number of digital channels is less than the real number of antennas, and the dimension of a channel matrix obtained by estimating the digital channel using the reference signal is far lower than the dimension of a channel matrix at the antenna end, so channel state information available in the digital domain cannot be used directly for the part of analog beam-forming. Furthermore since the reference signal corresponds to the digital channel, channel state information of the analog channels cannot be measured directly according to the reference signal in the case that the number of analog channels is inconsistent with the number of digital channels.